A pulsed radar system may be used to measure the distance to a target using the round-trip time of flight of the radar signal. The one-way distance, d, to the target is computed from the equation 2d=t*c where c is the velocity of light and where t is the time between the transmitted pulse and received pulse reflected from the target. Radar technology is well developed. However, an individual target may be difficult to isolate if there are many reflectors in the field of the radar. Also, the distance calculation may be corrupted by multi-path effects. Multi-path effects may be mitigated by using the time of the first return pulse from the target instead of the strongest return. However, this technique is fraught with problems if the reflected signal from the target is corrupted from signals reflected from other objects in the field of the radar system. RFID systems are well known in the art and are used to monitor objects and places by attaching a tag to objects and places to be monitored. These objects may be large and in the presence of many other reflecting objects. Backscatter RFID systems use CW signals, and thus lack the capability of ranging using pulsed signals. The phase of the backscattered signals from a modulated backscatter tag can be used to calculate the distance to a tag, as disclosed in provisional application Ser. No. 61/299,474 filed Jan. 29, 2010. In that system, accuracy may degrade severely in a highly reflecting environment due to multi-path effects. Time-of-flight methods to determine tag location may be found in the art. Most, if not all, of these are expensive and require careful installation, precisely locating the radar systems.
Many tens of millions of tags are presently in use. An object of the present invention is to enable ranging to tags that are already in use. Another object of the present invention is to determine the distance to a tag with an accuracy of one meter or better.
The present invention overcomes these difficulties and allows measurement of the distance to a modulated backscatter RFID tag in the presence of clutter and multi-path. The technique can be applied to other radar systems using the principles described below.
A modulated backscatter RFID tag transfers data from its memory to a remote reader by modulating the backscatter cross section of the tag antenna in a coded fashion, changing at a minimum from one reflecting state to another reflecting state (or between several reflecting states). A continuous wave (CW) radio signal is transmitted toward the tag by a reader. The tag modulates the reflected wave sent back to the reader. The reader then receives and decodes the modulated signal from the tag to extract the information sent by the tag. The decoding process recovers the timing of the modulation states of the tag. These timings cannot be used for ranging since there is no absolute time base to establish a reference time for ranging. (The art includes measuring the timing of pulses transmitted by a tag by several readers and calculating tag location using differences in timing between readers. This solution is complicated, expensive and inconvenient.) Also a practical problem is that the transitions from one state to the other lack the precision and definition in timing required for the nanosecond resolution required for ranging. For example, a resolution of 1 meter in tag location requires a timing resolution of 7 nanoseconds or better.
An example of the geometry of a conventional pulsed radar system is shown in FIG. 1. The radar system transmits a short pulse of RF signal which is reflected from the objects in the field of the radar and are received by the radar system. Strong multi-path signals may occur from a radio path bounced from the radar system to a flat surface (ground for example), to targets, and return. A sample plot of signals is shown in FIG. 2. The problem is to determine the time of the correct pulse of the many in FIG. 2.
All references cited herein are incorporated herein by reference in their entireties.